In semiconductor fabrication processes, the electrical properties of a semiconductor material, such as conductivity type, may be changed. Typically semiconductors can be doped with a donor impurity to change their conductivity to n-type where there is an excess of electrons or doped with an acceptor impurity to change their conductivity to p-type where there is an excess of holes.
A semiconductor is considered a wide bandgap semiconductor when it has an electronic bandgap EG that is significantly larger than three electron volts (i.e., EG≧3 eV). Wide bandgap semiconductors formed from group III metal nitride materials, such as aluminum nitride (AlN) or aluminum-gallium nitride (AlxGa1-xN), are also known as refractory materials due to their high formation temperatures. It is to be understood the term group III metal refers to atoms selected from the boron group of the periodic table of elements. Of particular interest are the group III metal nitride materials, which can be typically doped with a donor impurity specie, such as a group-IV atom specie (e.g. silicon (Si)), to change the conductivity to n-type. Conversely, group III metal nitrides are doped with an acceptor impurity, such as a group-II atom specie (e.g., magnesium (Mg)), to change the conductivity to p-type. Semiconductors formed from group III metal nitride materials are used in the fabrication of electronic devices, such as high power transistors, and optical devices, such as light-emitting diodes (LEDs).
One problem with using group III metal nitride materials in devices, is that in conventional device-fabrication processes, it is difficult to achieve a high level of activated doping density in group III metal nitride materials that have a relatively high aluminum (Al) content, such as AlxGa1-xN, where x is greater than about 0.5. It is further found that electronic grade levels of activated p-type doping (NA≧1018 cm−3) are particularly difficult to obtain in all group III metal nitride semiconductors, which is exacerbated for high Al % films. High Al % films of AlxGa1-xN and AlyIn1-yN for the case of x≧0.6 and y≧0.7, respectively, suffer a transition in the energy ordering of the valence bands in the vicinity of the crystal momentum zone center (i.e., in the vicinity of the direct gap). This further complicates the achievement of high levels of activated holes for electronic and optoelectronic devices.
Co-deposition techniques are typically used to incorporate a selected dopant impurity into a bulk AlxGa1-xN material. However, these techniques often do not achieve the desired activated doping densities. Dopant impurities co-deposited during the epitaxial formation of a bulk group III nitride film tend to either segregate to the surface of the growing film or are not readily incorporated into substitutional metal lattice sites of the group III metal nitride crystal structure. Consequently, the impurity dopants are not optimally incorporated into the host group III metal nitride material. It is found experimentally, in order to achieve sufficient electrically activated dopant concentrations in group III metal nitride films, adding relatively high levels of impurity atom dopants during epitaxial layer formation, is required to compensate for the poor incorporation of the said impurity dopants—typically exceeding the solubility limit. There is yet a further fundamental trade-off in achieving simultaneously the goals of (i) high electrically activated dopant concentrations and (ii) high quality crystal structure films. That is, high incident fluxes of dopant species during co-deposition are required to incorporate at least a portion (typically ˜1-10%) of the available impurity atoms into the growing group III metal nitride film, which comes at the disadvantageous expense of reduced structural quality of the resulting group III metal nitride film.
There is yet a further fundamental limitation of conventional impurity atom doping in wide band gap semiconductors, and particularly for group III metal nitride semiconductors. Even if ideal substitutional doping of a group III metal site within the host crystal is achieved, the activation energy of the donor or acceptor Eact(D,A) is generally large, and thus at room temperature only a small portion of the available excess carriers are activated within the host.
Light emitting devices are typically based on the quantum mechanical recombination of opposite carrier types, namely electrons and holes, within a given spatial region of a fixed bandgap semiconductor material. The optical bandgap defining the said recombination region controls the emitted optical energy of a photon created by the conversion of energy of the electron and hole. The supply of electrons and holes are provided by p-type and n-type reservoirs and typically configured in the layered structure of a PIN diode. The PIN diode comprises a p-type layer (hole reservoir), an intrinsic layer and an n-type layer (electron reservoir). The recombination process occurs substantially within the intrinsic region and is generally formed as a not intentionally doped (NID) semiconductor. By appropriate electrical bias, electrons and holes are simultaneously injected into the intrinsic layer of the PIN diode. It is understood that PN junction diodes can also be used.
Furthermore, when group III metal nitride materials are used in conventional fabrication of ultraviolet (UV) LEDs, the UV LEDs are constructed using multiple layers of dissimilar compositions of selected group III metal nitrides forming so called heterostructures. In heterostructure UV PIN or PN LEDs, a wider bandgap group III metal nitride material is used to form at least one of a p-type region and an n-type region of the LED, and a narrower bandgap group III metal nitride material is used to form an active recombination region of the LED. The wider bandgap region is required so as to provide a low absorption coupling external to the device. That is, the photogenerated light must be able to escape from the internal region of the LED.
One problem with heterostructure UV LEDs, is that at any group III metal nitride heterojunction, for example, the interface between the wider bandgap and narrower bandgap group III metal nitride material, creates extremely large internal polarization fields, (such as spontaneous polarization fields and piezoelectric fields). These internal polarization fields interfere with the distribution and transport of charge carriers, such as electrons and holes, within the active region of the LED, and consequently the recombination of carriers is substantially reduced by the extremely large internal electric polarization fields which tend to inhibit the ideal spatial localization of electron and holes. In fact, the built in electric polarization fields tend to spatially separate the electron and hole wavefunctions in the recombination region. That is, the electronic spatial probability distributions (known by workers in the field as quantum mechanical wavefunctions) of the electrons and holes are not aligned and thus the so called overlap integral is severely diminished and thus recombination is substantially reduced. The amount of light emitted from the LED is therefore substantially reduced compared to the equivalent case where internal electric polarization fields do not exist.